1. Field of the Invention
This invention relates to an apparatus for measuring a visual line length, a distance from a first objective surface of an eye ball to a second objective surface, and particularly to an apparatus for measuring an eye axial length, the depth of an anterior chamber, the thickness of the crystal lens, etc. in a noncontact style.
2. Description of the Prior Art
Heretofore, there has been an apparatus using a supersonic wave as an apparatus for measuring a visual line length as a distance from a first anterior face of an eye ball to a second rear face thereof an eye axial length, the depth of an anterior chamber, the thickness of a crystal lens, etc. The supersonic wave is projected toward the eye.
And this supersonic wave is reflected by an anterior face of cornea, an anterior face of crystal lens, a rear face of crystal lens, and a surface of retina. These echoes are drawn on a Braun tube or CRT. Echogram drawn on the CRT is taken for measurement.
However, as the measurement accuracy of the visual line length of this conventional apparatus is about .+-.0.2 mm, such measured value is not enough to use for determining power of, for example, IOL (intraocular lens).
Also, as a probe is contacted with the eye when the eye is measured with this conventional apparatus using a supersonic wavelength is necessary to take some measures for avoiding infection.
Therefore, in a recent year, there was proposed an apparatus for measuring the visual line length, in a noncontact style by observing an interference fringe.
One such example is depicted in FIG. 6. This apparatus is used for measuring the visual line length. This apparatus is described in A. F. Fercher et al. (OPTICS LETTER VOL. 13 NO.3 PP.186-188 (MAR. 1988) Optical Society of America).
The apparatus of FIG. 6 generally comprises a semiconductor laser 1, a collimate lens 2, a couple of parallel planes 3, 4, a beam splitter 5, a condenser lens 6, and an image pick up device type camera 7. Laser beam emitted from the semiconductor laser 1 is made into a parallel flux of light rays by the collimate lens 2. The parallel flux of light rays pass through the couple of parallel planes 3, 4. The parallel flux of light rays (hereinafter referred to as "light flux 1"), which have passed through the couple of parallel planes 3, 4 are guided to an eye 8 through a beam splitter 5. And the light flux 1 are made into a convergent light by the function of the eye 8. The convergrent light reaches a retina 9. This convergent light is reflected by the retina 9. The reflected light is emitted from the eye 8 in the form of generally parallel flux of light rays (plane wave). The light flux 1 coming from the eye 8 is reflected by a reflecting surface 10 of the beam splitter 5 in the direction where the condenser lens 6 is placed. The condenser lens 6 condenses the light flux reflected by the reflecting surface 10. The light flux passed through the condenser lens 6 reaches the image pick up device type camera 7.
Also, a part of the light flux 1 passed through the parallel plane 3 is reflected by the parallel plane 4. The reflected light flux (hereinafter referred to as the "light flux" 2) are returned to the parallel plane 3 and passed through this parallel plane 4. The light flux 2 passed through the parallel plane 4 are passed through the beam splitter 5 and guided to a cornea 11. Then, the flux 2 are reflected by the cornea 11. The reflected light from the cornea 11 is guided to the beam splitter 5 in the form of divergence beam (spherical wave). The divergence beam is reflected by the reflecting surface 10 to reach the condenser lens 6. The divergence beam is condensed by the condenser lens 6 and reaches the camera 7. In FIG. 6, the numeral 12 denotes a sensor for monitoring amount of light of the semiconductor laser 1.
In this conventional apparatus, a distance l between the parallel planes 3 and 4 can be changed. If the refractive index of a substance existing between the parallel planes 3 and 4 is represented by n, the refractive index of an intraocular material is represented by N, and the measured value (distance from the apex of the cornea 11 to the retina 9) of the visual line length is represented by X, the distance l between the parallel planes 3 and 4 is adjusted in such a manner as to satisfy the following equation. EQU n.multidot.l=N.multidot.X
Then, an optical path length of the light flux 1 and an optical path length of the light flux 2 become equal.
Therefore, the interference fringe is observed by using the camera 7. By obtaining the position (distance l) of the parallel plane as a measured value at the time when this interference fringe is observed, the visual line length X can be found.
The apparatus for measuring the visual line length by observing this interference fringe has light flux reflected from the outer surface of the cornea is a generally spherical wave. On the other hand, the light flux reflected from the the surface of the retina is a generally plane wave. Therefore, the number of stripes of the interference fringe is greatly increased as it goes away to the marginal portion from the apex of the cornea and the interference fringe cannot be favorably observed. Furthermore, in this conventional apparatus, it is very troublesome in order to align the optical axes of the condenser lens 6 and the camera 7 with respect to the eye 8.